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Modernizing insulation in a 1900’s balloon-framed house from the exterior during siding replacement?

ericwages | Posted in Green Building Techniques on

Hi folks!

I’m seeking the collective wisdom from the community to figure out what the best compromise may be to modernize our 1900’s balloon-framed house in the northern suburbs of Boston (Zone 5). Best we are aware, there is no insulation in any of the walls and is sheathed on the exterior with asbestos shingles, some form of reflective underlayment that I can see peeking out through gaps, and normal dimensional lumber underlayment. Interior, we have plaster and lath walls and it appears all original knob-and-tube has been replaced with either MC or modern Romex electrical runs.

Due to a number of issues, we are looking to have a tear-off done from the outside down to the studs to inspect and repair and rot/bug damage prior to residing and insulating the house. This would be the opportune time to insulate and modernize the wall assembly – within reason and doing work at this time from the inside out isn’t viable. We plan on being here a very long time so we’re more interested in doing it right than easy. The attic is used for storage and has a covered plank floor and batt insulation is visible through the gaps in the boards. Unknown depth, but at least 8-10″. Long term, the attic roof will be re-insulated and making the whole volume heated.

Here’s the core question: what would you all recommend for this outside-in scenario for the walls? I can see a number of avenues that have varying efficacy and all pivot around the reality of not being able to install a vapor barrier in the traditional most inward location. I’m certainly no expert, but trying to think through the various possibilities to steer the conversation with the vendor-to-be-hired:

Option a) Rockwool Comfortbatt (R15) between the studs with either a insulated Zip-R panel, furring for a proper rainscreen gap, and then vinyl siding. Provides dramatically-improved insulation, fireproofing, and is hygrophobic and can get up to modern insulation values. May need to change/extend window sills. No direct vapor barrier though.

Option b) A flash and batt from the outside to make a solid air seal against the backside of the lath. Then similar insulated zip-R (or other), rainscreen, and siding. No <R15 Comfortbatt available in the US, so this would probably have to be a traditional fiberglass solution. 

Option c) Something else completely or a modification of above? 

Am I just completely overthinking this or out in left field? I appreciate the incoming input.

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  1. nhbean | | #1


    I've been upgrading a similar-age balloon-framed house in Zone 4, also from the outside. I'll share our process, which may be helpful. We had siding over board sheathing - I suspect you'll find that you also have board sheathing under your asbestos shingles. Even though I did have some rot issues (one whole corner of the house had dry rot due to bulk water intrusion from a missing kickout flashing, which was the cause of the project), it proved possible to inspect the framing without removing much of the sheathing.

    We 1) removed the siding, 2) repaired any rot/damage we found, 3) air-sealed the sheathing using Grace Vycor Env-S (a peel-and-stick WRB) as the air barrier, 4) added two layers of 1" reclaimed foil-faced polyiso with taped seams, 5) screwed 1x4" furring strips through the foam to provide a rainscreen, and 6) nailed new siding to the furring strips.

    This approach does require extending the window sills. You can also insulate the wall cavities, if you choose - just make sure your exterior foam is thick enough to prevent condensation in the walls (ours were already filled with cellulose, and we added more to bays that were missed when the insulation work was originally done). I was worried about hitting studs through the foam, but there is a definite difference you feel when driving the screws into a stud or just sheathing, and we only had to go "fishing" for a stud a few times.

    Also, pay close attention to the roof eaves - ours were formed by the attic floor joists extending beyond the wall, and they were supporting a kicked roof. The fascia and soffit board were nailed directly to these, leaving the joist space completely unsealed and uninsulated - we are installing blocking and then spray foaming against that. The transition to the foundation is also something you need to pay careful attention to.

    Let me know if you have any questions. I'd be happy to share my experiences.

  2. Expert Member
    Akos | | #2

    The most important part of insulating balloon framed houses is air sealing. Lot of times the wall cavity is open to the ceiling, floor, basement and attic creating a huge air leak from stack effect.

    Usually the best way to deal with this is to dense pack the walls. This can be done from the outside either through the original sheathing or through the new sheathing if you are replacing it all.

    SPF can also work but much more expensive. Sometimes it is hard to find someone local to dense pack, in which case SPF is good. You don't need closed cell, open cell foam works just as well.

    A cellulose filled wall is pretty safe in your climate especially since a building that old probably has many layers of oil paint on the wall making for a decent vapor barrier.

    With these old houses, the item to watch for is flashing details around your windows and doors. These are typically non-existent and unless they are fixed, you'll end up with a moldy insulation mess in the walls.

    If you are keeping the board sheathing, covering the whole house with a permeable peel and stick WRB will further tighten up the assembly, or if you are installing new sheathing, make sure to tape the seams. A rain screen under the new siding is also a good thing.

    This is also the time to install exterior insulation if you really want to cut down your energy use, but this can add a fair bit to a project especially if local trades are not familiar with it.

    P.S. Batts with older real 2x4 studs are a pain, generally something I try to avoid as they don't fit well and not thick enough to fill the cavity.

    P.S.S. The old sheathing is proving the racking strength to the house, make sure to get an engineer to look at it before removing it as you might need to temporarily brace the walls.

  3. ericwages | | #3

    Thanks for the tips - keep 'em coming. We're hoping to solidify on a solution so we can get this project underway!

    I'll be handling the air sealing in the basement and attic - easily done in this case as it's easily reached in both the attic and basement.

    I'm confident the flashing is done poorly - it all needs to come off when the asbestos comes off even though the previous owner replaced the windows 10 years ago. He just did a bad job of it. Same with soffits and other trim work.

    So is the general consensus to just avoid even trying to apply direct batt insulation? We're hoping to both improve insulation *and* some level of fire resistance/ water resistance inside of the assembly. Or is it, in your experience, that the on-center measurements of old dimensional framing is just too inconsistent to reliably use modern batts?

  4. Patrick_OSullivan | | #4

    I've done almost exactly this. I have a 1916 home in NJ (4A) and while doing some addition work, opted to add Zip R over the entire house. I had the benefit of replacing all windows at the same time, so I didn't have to worry about changing the exterior plane.

    It was a bit of a ballet to get it done, but our relative order of operations was:

    1. Strip shingles (in my case, cedar)
    2. Dense pack from outside (and a little from inside, depending on accessibility)
    3. Remove windows
    4. Sheath with Zip R over existing sheathing

    There really wasn't a need to remove existing sheathing. As was mentioned by others, that existing sheathing is providing rigidity and disturbing it would have been more trouble than it's worth.

  5. walta100 | | #5

    Your plaster may be in better shape than the old house I worked on but I am sure any use of a hammer on most studs would have would have cracked the plaster and several whacks would likely bring down big chunks of plaster.

    Are you in a historic district? Some can be very picky about tiny details. Making a wall an inch thicker for zip R does not sound hard until the historic commission requires the same window set back and trim as you have now.

    Adding fire blocking in each stud bay every 8 feet is a good idea and often enforced code requirement.


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